is there any way to watch tv full screen. i have my video set at 720p and myth tv shows 4:3 with black bars on either side

I'm not familiar with Myth... but are the TV shows broadcast in 4:3 or 16:9? If they were broadcast in 4:3 then Myth is just showing them as they are broadcast. I assume Myth has some setting that will allow you to stretch/crop the 4:3 picture to 16:9 though.